The Assembly State between Magnetic Nanosensors and Their Targets Orchestrates Their Magnetic Relaxation Response

The target-induced clustering of magnetic nanoparticles is typically used for the identification of clinically relevant targets and events. A decrease in the water proton transverse NMR relaxation time, or T-2, is observed upon clustering, allowing the sensitive and accurate detection of target molecules. We have discovered a new mechanistically unique nanoparticle-target interaction resulting in a T-2 increase and demonstrate herein that this increase, and its associated r(2) relaxivity decrease, are also observed upon the interaction of the nanoparticles with ligands or molecular entities. Small molecules, proteins, and a 15-bp nucleic acid sequence were chemically conjugated to polyacrylic-acid-coated iron oxide nanoparticles, and all decreased the original nanoparticle r(2) value. Further experiments established that the r(2) decrease was inversely proportional to the number of ligands bound to the nanoparticle and the molecular weight of the bound ligand. Additional experiments revealed that the T-2-increasing mechanism was kinetically faster than the conventional clustering mechanism. Most importantly, under conditions that result in T-2 increases, as little as 5.3 fmol of Bacillus anthracis plasmid DNA (pX01 and pX02), 8 pmol of the cholera toxin B subunit (Ctb), and even a few cancer cells in blood were detected. Transition from the binding to the clustering mechanism was observed in the carbohydrate-, Ctb-, and DNA-sensing systems, simply by increasing the target concentration significantly above the nanoparticle concentration, or using Ctb in its pentameric form as opposed to its monomer. Collectively, these results demonstrate that the molecular architectures resulting from the interaction between magnetic nanosensors and their targets directly govern water proton NMR relaxation. We attribute the observed T-2 increases to the bound target molecules partially obstructing the diffusion of solvent water molecules through the superparamagnetic iron oxide nanoparticles' outer relaxation spheres. Finally, we anticipate that this novel interaction can be incorporated into new clinical and field detection applications, due to its faster kinetics relative to the conventional nanoparticle-clustering assays.